Videos: the clip just left keeps its player, so a swipe back starts at once too (on Adam's phone it was the slow one: 0.66-0.78 s, built anew, against 0.04-0.08 s onto the warm next clip). The swipe clock records the player's readyState and how far it had buffered.

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_013ZDf1cyCQLkfjftYhymZAg

2 files changed, 22 insertions, 10 deletions

diff --git a/src/features/messages.js b/src/features/messages.js
index dadd084..bf6096a 100644
--- a/src/features/messages.js
+++ b/src/features/messages.js
@@ -14516,6 +14516,7 @@ export function messagesFeature(ctx) {
     const prevGroup = (g) => { let n = g && g.previousElementSibling; while (n && (!n.classList.contains('mp-group') || n.classList.contains('mp-off'))) n = n.previousElementSibling; return n; };
     const up = prevGroup(slide.closest('.mp-group')), back = up && rowSlide(up);
     if (back) out.push(back); // (the one before: ready again for the swipe back)
+    out.back = back || null;
     return out;
   }
   function pagerAttach(slide) {
@@ -14568,16 +14569,20 @@ export function messagesFeature(ctx) {
       if (j === i) return;
       const v = s.querySelector('video.mp-media');
       if (eager && k >= 0) {
-        // the next clip: a player of its own, warm — once its opening is in
-        // the cache, given the file to read up to its first frame (preload
-        // 'metadata': Chrome reads ~2 s of it and stops) and paused there,
-        // so the swipe onto it is only play and sound (TikTok's trick). The
-        // others ahead get their opening only.
-        if (k === 0 && v.getAttribute('src')) return; // warm already
+        // the next clip and the one before: a player of its own, warm — once
+        // its opening is in the cache, given the file to read up to its
+        // first frame (preload 'metadata': Chrome reads ~2 s of it and
+        // stops) and paused there, so the swipe onto it is only play and
+        // sound (TikTok's trick). The one just left keeps the player it
+        // played in (a swipe back to it took 0.7 s on a phone, built anew).
+        // The others ahead get their opening only.
+        const warm = k === 0 || s === ahead.back;
+        if (warm && v.getAttribute('src')) return; // warm already
         pagerRelease(s);
         const head = headStart(playSrc(v.getAttribute('data-src')), k === 0 ? 1 : 0.5);
-        if (k === 0) head.then(() => {
-          if (!pager || !s.isConnected || pager.cur === s || v.getAttribute('src') || pagerAhead(pager.cur)[0] !== s) return;
+        if (warm) head.then(() => {
+          const now = pager && pager.cur && pagerAhead(pager.cur);
+          if (!now || !s.isConnected || pager.cur === s || v.getAttribute('src') || (now[0] !== s && now.back !== s)) return;
           v.preload = 'metadata';
           v.muted = true;
           pagerAttach(s);
@@ -14798,12 +14803,13 @@ export function messagesFeature(ctx) {
     const t0 = slide._arriveAt;
     slide._arriveAt = 0;
     if (!t0) return;
-    const url = v.getAttribute('data-src') || '', warm = v.readyState >= 2;
+    const url = v.getAttribute('data-src') || '', warm = v.readyState >= 2, rs = v.readyState;
+    const buf = v.buffered && v.buffered.length ? +v.buffered.end(v.buffered.length - 1).toFixed(1) : 0;
     const d = videoInfo.get(url) || {};
     let host = ''; try { host = new URL(url).hostname; } catch {}
     const done = () => {
       const ms = Math.round(performance.now() - t0);
-      pagerStarts.push({ ms, warm, host, mb: d.size ? +(d.size / 1e6).toFixed(1) : null, faststart: d.faststart ?? null, src: (v.getAttribute('src') || '').startsWith(VMEDIA) ? 'copy' : 'original', url });
+      pagerStarts.push({ ms, warm, rs, buf, host, mb: d.size ? +(d.size / 1e6).toFixed(1) : null, faststart: d.faststart ?? null, src: (v.getAttribute('src') || '').startsWith(VMEDIA) ? 'copy' : 'original', url });
       if (pagerStarts.length > 100) pagerStarts.shift();
       dlog('pager start', ms + ' ms', warm ? 'warm' : 'cold', host);
     };
diff --git a/tools/pager-eager-test.js b/tools/pager-eager-test.js
index 01c8dd6..3b5c5c3 100644
--- a/tools/pager-eager-test.js
+++ b/tools/pager-eager-test.js
@@ -151,6 +151,12 @@ async function run(saveData, slow = false) {
       // the link measured now: a fast one, still a couple of seconds' worth; one slower than the clip, the cap
       if (!slow) check('…a fast link measured: still two seconds\' worth (half, as the second ahead)', near(head3, RATE), kb(head3));
       else check('…the link measured slower than the clip: it grows to the cap (half, as the second ahead)', head3 === HEAD_MAX / 2, kb(head3));
+      // the clip just left keeps its player: the swipe back is warm too
+      check('the clip just left keeps its player', !!(await slide('c0'))?.src);
+      await page.keyboard.press('ArrowUp');
+      await waitFor(() => coinosPagerStarts().starts.filter((x) => /c0\.webm$/.test(x.url)).length >= 2, null, 5000);
+      const back = await page.evaluate(() => coinosPagerStarts().starts.filter((x) => /c0\.webm$/.test(x.url)).at(-1));
+      check('…so swiping back to it starts warm', back && back.warm && back.ms < 1000, JSON.stringify(back));
     } else {
       await sleep(3000);
       const s = await slide('c1');